The profile of the interface state densities(N ss) and series resistances (R s) effect on capacitance-voltage (C-V) and conductancevoltage (G/ω-V) of (Ni/Au)/Al xGa 1-xN/AIN/ GaN heterostructures as a function of the temperature have been investigated at 1 MHz. The admittance method allows us to obtain the parameters characterizing the metal/semiconductor interface phenomena as well as the bulk phenomena. The method revealed that the density of interface states decreases with increasing temperature. Such a behavior of N ss can be attributed to reordering and restructure of surface charges. The value of series R s decreases with decreasing temperature. This behavior of R s is in obvious disagreement with that reported in the literature. It is found that the N ss and R s of the structure are important parameters that strongly influence the electrical parameters of (Ni/Au)/Al xGa 1-XN/AlN/GaN(x = 0.22) heterostructures. In addition, in the forward bias region a negative contribution to the capacitance C has been observed, that decreases with the increasing temperature. Copyright © 2010 John Wiley &amp; Sons, Ltd

Aydemir, U.

Taşçloǧlu I.

Özbay, E.

Şafak, Y.

Bilkent University Institutional Repository

812Research ArticleReceived: 27 August 2009 Revised: 2 December 2009 Accepted: 11 January 2010 Published online in Wiley Interscience: 28 March 2010(www.interscience.wiley.com) DOI 10.1002/sia.3249Temperature-dependent profile of the surfacestates and series resistance in (Ni/Au)/AlGaN/AlN/GaN heterostructures†İ. Taşçıoǧlu,a U. Aydemir,a Y. Şafaka∗ and E. ÖzbaybThe profile of the interface state densities (Nss) and series resistances (Rs) effect on capacitance–voltage (C –V) and conductance-voltage (G/ω–V) of (Ni/Au)/Alx Ga1−xN/AlN/GaN heterostructures as a function of the temperature have been investigatedat 1 MHz. The admittance method allows us to obtain the parameters characterizing the metal/semiconductor interfacephenomena as well as the bulk phenomena. The method revealed that the density of interface states decreases with increasingtemperature. Such a behavior of Nss can be attributed to reordering and restructure of surface charges. The value of series Rsdecreases with decreasing temperature. This behavior of Rs is in obvious disagreement with that reported in the literature.It is found that the Nss and Rs of the structure are important parameters that strongly influence the electrical parameters of(Ni/Au)/Alx Ga1−xN/AlN/GaN(x = 0.22) heterostructures. In addition, in the forward bias region a negative contribution to thecapacitance C has been observed, that decreases with the increasing temperature. Copyright c© 2010 John Wiley & Sons, Ltd.Keywords: (Ni/Au)/AlGaN/AlN/GaN heterostructures; interface states; negative capacitance; frequency dependence; series resistanceIntroductionThe performance and reliability of electronic devices suchas metal–semiconductor (MS), metal–insulator–semiconductor(MIS), metal–ferroelectric–semiconductor (MFS) or metal–ferroelectric–insulator–semiconductor (MFIS), metal–oxide–semiconductor field effect transistor (MOSFET) and high elec-tron mobility transistors (HEMTs) are dependent especially on theformation barrier height at M/S interface, and series resistance(Rs) of devices, doping concentration and density of interfacestates (Nss).[1 – 6] Also, change in temperature has very importanteffects on the parameters of such devices.[7 – 9] The existence ofan interfacial insulator layer at M/S interface and the Rs of the de-vice significantly alter the device’s capacitance–voltage (C –V)and conductance–voltage (G/ω–V) characteristics. In recentyears, some investigations have reported a negative capacitance(NC)[1,2,4,5,10 – 12] in the forward bias C –V characteristic. The termNC means that the material displays an inductive behavior. Theobservation of NC is important because it implies that an incre-ment of bias voltage produces a decrease in the charge on theelectrodes.[4] However, NC has, so far, no meaning to us and theconcept of NC is still not widely recognized because of lack of trustin experimental data.[12] Therefore, in many cases experimental NCdata were not reported in the literature due to confusion causedby the NC effect.[13] Practically, NC can be explained based on thebehavior of temperature and frequency dependent admittancespectroscopy (C –V and G/ω–V) data.[12] As the electrons thatsurmount the Schottky barrier (SB) under forward bias do fill upthe empty states at the interface and possess excess energy, whencolliding with the electrons trapped at the Nss they could alsoknock electrons out of the traps, provided the binding energy ofthese traps is smaller than the SB energy.[2,13]Both the Nss and Rs values of these devices are importantparameters that affect the main electrical parameters.[14] Since abias voltage is applied across these structures, the combination ofthe interfacial insulator layer, depletion layer and series resistanceof the device will share the applied bias voltage. The Nss andbulk traps formed at M/S interface, where charges can be storedand released when the appropriate forward applied bias and theexternal a.c. oscillation voltage are applied, strongly affect deviceperformance.[1,2,4,5,11] Although it is believed that the injectionof charge carriers involves a process of hopping to localizedinterface traps, a detailed physical mechanism of injection is notyet understood.In this study, the origin of NC in the forward bias C –V characteris-tics of (Ni/Au)/Alx Ga1−xN/AlN/GaN(x = 0.22) heterojunctions hasbeen investigated between 80 and 400 K at 1 MHz. Experimentalresults show that the value of NC decreases with increasing tem-perature at forward bias voltage and this decrease correspondsto an increase of the conductance. In addition, the temperature-dependent profiles of Nss and Rs were found at each temperatureby using the Hill–Coleman[15] and conductance[16] methods, re-spectively.ExperimentalThe AlxGa1−xN/AlN/GaN (x = 0.22) heterostructures were grownon a double-polished 2-inch diameter (0001) sapphire (Al2O3) sub-strate in a low pressure Metal-Organic Chemical-Vapor Deposition∗ Correspondence to: Y. Şafak, Department of Physics, Gazi University, 06500Ankara, Turkey. E-mail: yaseminsafak@gazi.edu.tr† Paper published as part of the ECASIA 2009 special issue.a Department of Physics, Gazi University, 06500 Ankara, Turkeyb Department of Physics, Nanotechnology Research Center, Bilkent University,06800 Ankara, TurkeySurf. Interface Anal. 2010, 42, 812–815 Copyright c© 2010 John Wiley & Sons, Ltd.813Temperature-dependent profile of the surface states and series resistance(MOCVD) reactor (Aixtron 200/4 HT-S) by using trimethylgallium(TMGa), trimethylaluminum (TMAl), and ammonia as Ga, Al, and Nprecursors, respectively. Prior to the epitaxial growth, Al2O3 sub-strate was annealed at 1100 ◦C in a high vacuum under a pressureof 10−7 Torr for 10 min in order to remove surface contamination.In order to design AlGaN/AlN/GaN heterojunction structures, wehave grown GaN, AlN and AlGaN layers on a sapphire substrate.Because of large lattice and thermal mismatch we used bufferlayers between substrate and GaN layer. In AlGaN/AlN/GaN het-erojunction, the AlGaN, AlN and GaN layers are the barrier, spikeand buffer layers, respectively. The buffer structures consisted ofa 15-nm-thick, low-temperature (650 ◦C) AlN nucleation layer andhigh-temperature (1150 ◦C) 420-nm AlN templates. A 1.5-µm nom-inally undoped GaN layer was grown on an AlN template layer at1050 ◦C, followed by a 2-nm-thick high-temperature AlN (1150 ◦C)barrier layer. After the deposition of these layers, a 23-nm-thickundoped Al0.22Ga0.78N layer was grown on an AlN layer at 1050◦C.Finally, a 5-nm-thick GaN cap layer growth was carried out at atemperature of 1085 ◦C and a pressure of 50 mbars.The ohmic contacts were formed as a square van de Pauwshape and the Schottky contacts formed as 1 mm diametercircular dots.[17] Prior to ohmic contact formation, the sampleswere cleaned with acetone in an ultrasonic bath. Then, the samplewas treated with boiling isopropyl alcohol for 5 min and rinsed indeionized (DI) water having 18 M resistivity. After cleaning, thesamples were dipped in a solution of HCl/H2O (1 : 2) for 30 s in orderto remove the surface oxides and then rinsed in DI water again for aprolonged period. Ti/Al/Ni/Au (17.5/175/40/150 nm) metals werethermally evaporated on the sample and annealed at 850 ◦C for 30 sin ambient N2 in order to form the ohmic contact. In order to obtaina rectifying/Schottky contacts, Ni/Au (40/80 nm) layer was coatedon the top of sample in the high vacuum coating system at about10−7 Torr. The schematic diagram of (Ni–Au)/AlGaN/AlN/GaNheterostructures has been already published.[17]The C –V –T and G/ω–V –T measurements of the (Ni/Au)/AlxGa1−xN/AlN/GaN(x = 0.22) heterostructures were taken usinga computer controlled HP 4192 A LF impedance analyzer at1 MHz. The sample temperature was controlled using Janes vpf-475 cryostat, which enables us to take measurements in thetemperature range of 77–450 K. Also, the sample temperature wascontinually monitored by using a copper-constant thermocoupleclose to the sample and measured with a Keithley model199 DMM/scanner and a Lake Shore model 321 auto-tuningtemperature controller with sensitivity better than ±0.1 K.Results and DiscussionIt is well known that the analysis of the C –V and G/ω–Vcharacteristics of semiconductor devices such as MS, MIS, MOSand HEMTs only at room or narrow temperature range cannotgive us detailed information about the main electrical parametersand/or conduction mechanisms. However, the C –V and G/ω–Vmeasurements of these devices in the wide temperature andbias voltage regions allows us to understand different aspectsof conduction mechanisms or the temperature and bias voltagedependence behavior of main electrical parameters. Therefore,we have investigated the C –V and G/ω–V characteristics of(Ni/Au)/Alx Ga1−xN/AlN/GaN(x = 0.22) heterostructures in thewide temperature range of 80–400 K and at 1 MHz. Figure 1(a)and (b) shows the plots of the measured C –V and G/ω–V of(Ni/Au)/Alx Ga1−xN/AlN/GaN heterostructure in the temperaturerange 80–400 K.Figure 1. The temperature-dependent plots of (a) the C –V and (b) G/ω–Vcharacteristics for the (Ni/Au)/Alx Ga1−x N/AlN/GaN heterostructure mea-sured at 1 MHz.As shown in Fig. 1(a) and (b), both the values of C andG/ω decrease with increasing temperature especially in theaccumulation and depletion regions for each bias voltage. Inaddition, an interesting feature of the forward bias C –V curvesis the nearly common intersection point (at about 5.5 V) of allthe curves at a certain bias voltage, and for this voltage point, theconduction through the junction is temperature independent. Theintersection behavior of the C –V curves appears abnormal whencompared to the conventional behavior of ideal Schottky diodesand MIS structures. This behavior is attributed to the lack of freecharge at low temperature and in the region, where there is nocarrier freezing out, which is nonnegligible at low temperature, inparticular. On the other hand, the existence of Rs in these structurescauses bending due to charge saturation, and plays a subtle rolein keeping this intersection hidden.[14] When the temperature isincreased, the generation of thermal carriers (electrons or holes) insemiconductor is enhanced both at positive and negative biasedconditions. Therefore, the increase of C with the temperature forSurf. Interface Anal. 2010, 42, 812–815 Copyright c© 2010 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/sia814İ. Taşçıoǧlu et al.Figure 2. The temperature-dependent Rs for the (Ni/Au)/Alx Ga1−x N/AlN/GaN heterostructure measured at 1 MHz.all applied bias levels can be understood due to charge storage(= Q/V). The other interesting feature, as can be seen in Fig. 1(a), isthat the values of capacitance give two peaks. The first peak in thedepletion region due to interface states (Nss) contribution shiftsfrom the negative bias voltage to the forward bias voltage as thetemperature increases. However, the second peak shown in theaccumulation region disappears when the temperature increases.Such a behavior of the second peak is due to the influence of theseries resistance (Rs) of the heterostructures. While the values ofNss are effective especially in the depletion and weak inversionregions, the values of Rs are effective only in the accumulationregion.In addition, C –V –T curves show negative values after a certainforward bias voltage (∼5.5 V). Contrary to the C –V curves, thevalues of G/ω increase from the inversion region to the strongaccumulation region at each temperature. As shown in Fig. 1(a)and (b), the NC values appear especially at low temperatures(T ≤ 260 K) and at high temperatures (T > 260 K) they disappear.Also, as can be seen in Fig. 1(b), the NC values correspondto the maximum of the device conductance. Such a behaviorof C –V –T and G/ω–V –T shows that the material displays aninductive behavior.[11,12] It is believed that the injection of chargecarriers involves a process of hopping to localized interfacetraps/states, but detailed physical mechanisms of injection arenot yet understood.Temperature dependence of Rs can be determined from themeasurements of C –V –T and G/ω–V –T curves at sufficientlyhigh frequency as[16]Rs = GmG2m + (ωCm)2(1)where Cm and Gm represent the measured capacitance andconductance for any bias voltage, respectively. Figure 2 showsthe variation of the series resistance as a function of temperature.These very significant values demand that special attention shouldbe given to effect of the Rs in the application of the frequencyand temperature-dependent C –V and G/ω–V measurements.It is clearly seen in Fig. 2 that the values of Rs decrease withTable 1. The values of different parameters for a(Ni/Au)/AlxGa1−x N/AlN/GaN heterostructure determined fromC –V and G/ω–V characteristics in the temperature range 80–400 KT (K) Vmax (V) Cmax (F) (Gm/ω)max (F) Nss (eV−1 cm−2)80 −7.9 7.06E−10 8.11E−10 1.76E+12110 −7.7 9.10E−10 9.94E−10 2.38E+12140 −7.4 9.50E−10 1.05E−09 2.56E+12170 −7.3 8.50E−10 9.85E−10 2.29E+12200 −7 7.60E−10 9.30E−10 2.07E+12230 −7.1 6.50E−10 7.05E−10 1.49E+12260 −6.9 5.48E−10 6.36E−10 1.28E+12290 −7.1 4.50E−10 5.39E−10 1.04E+12320 −7.2 3.82E−10 4.42E−10 8.29E+11340 −7.4 3.43E−10 3.79E−10 6.99E+11360 −7.5 3.07E−10 3.45E−10 6.27E+11380 −7.6 2.78E−10 3.20E−10 5.74E+11400 −7.8 2.46E−10 3.03E−10 5.36E+11the increasing bias voltage from strong inversion region toaccumulation region. Also, the values of Rs increase with increasingtemperature both in the accumulation and depletion regions.Such a temperature dependence of Rs is in obvious disagreementwith the reported negative temperature coefficient of the idealShottky barrier diodes (SBDs) and MIS type SBDs. Similar resultshave been reported in the literature.[18] This change in the Rswith the temperature can be expected for semiconductors in thetemperature region where there is no freezing behavior of thecarriers. We believe that the trap charges have sufficient energyto escape from the traps that are located between the metal andsemiconductor interface.It is well known that the density of interface states (Nss) is auseful guide for the quality of semiconductor devices such as SBDsand HEMTs structures. There are several methods to determineNss.[15,16,19] Among them, Hill–Coleman method[15] is important interms of being fast and reliable. According to this method, densityof interface states can be expressed asNss = 2qA(Gm/ω)max((Gm/ω)maxCox)2 + (1 − Cm/Cox)2), (2)where A is the area of rectifier contact, ω is the angular frequency,Cm and (Gm/ω)max are the measured capacitance and conductancewhich correspond to the peak values, respectively, and Cox is thecapacitance of insulator layer. The values of various parameters fora (Ni/Au)/Alx Ga1−xN/AlN/GaN heterostructure determined fromC –V and G/ω–V characteristics in the temperature range of80–400 K are given in Table 1 and Figs (3) and (4), respectively.As shown in Figs (3) and (4) the values of Cmax, G/ωmax and Nssgive a peak at about 140 K. This is a result of molecular restructuringand reordering of the metal–semiconductor interface caused bythe temperature.[20]ConclusionsThe temperature-dependent profiles of the Nss and Rs were ob-tained from forward and reverse bias C –V –T and G/ω–V –Tcharacteristics of (Ni/Au)/Alx Ga1−xN/AlN/GaN(x = 0.22) het-erostructures in the wide temperature range of 80–400 K at 1 MHz.www.interscience.wiley.com/journal/sia Copyright c© 2010 John Wiley & Sons, Ltd. Surf. Interface Anal. 2010, 42, 812–815815Temperature-dependent profile of the surface states and series resistanceFigure 3. The variation of the Cmax and (Gm/ω)max for (Ni/Au)/Alx Ga1−x N/AlN/GaN heterostructures as a function of the temperatureat 1 MHz.Figure 4. The variation of the Nss for (Ni/Au)/Alx Ga1−x N/AlN/GaN het-erostructures as a function of the temperature at 1 MHz.It was found that both the C and G/ω were quite sensitive to tem-perature, especially at relatively low temperature. The value ofseries Rs decreases with decreasing temperature. This behaviorof Rs is in obvious disagreement with that reported in literatureThe value of Nss gives a peak at about 140 K due to molecular re-structuring and reordering of the interface states and dislocationsbetween the metal and the semiconductor. In addition, the C –Vplots cross at certain forward bias voltage points (∼5.5 V) and thenshow negative values. The intersection behaviors of C –V curvesand the increase in Rs with temperature were attributed to thelack of free charge especially at a low temperature. The value ofNC decreases with increasing temperature at forward bias voltageand this decrease of the NC corresponds to an increase of theconductance. It is thought that the capacitance value decreaseswith increasing polarization and more carriers are introduced inthe structure. It is found that the Nss and Rs of the structureare important parameters that strongly influence the electricalparameters of (Ni/Au)/Alx Ga1−xN/AlN/GaN heterostructures.References[1] J. Bisquert, G. Garcia-Belmonte, A. Pitarch, H. J. Bolink, Phys. Lett.2006, 422, 184.[2] S. Salahuddin, S. Datta, Nano Lett. 2008, 8(2), 406.[3] F. El Kamel, P. Gonon, F. Jomni, B. Yangui, Appl. Phys. Lett. 2008, 93,042904.[4] C. Y. Zhu, L. F. Feng, C. D. Wang, H. X. Cong, G. Y. Zhang, Z. J. Yang,Z. Z. Chen, Solid State Electron. 2006, 53, 324.[5] A. H. Jayatissa, Z. Li, Mater. Sci. Eng. B 2005, 124–125, 331.[6] Ş. Altindal, H. Kanbur, Y. Yucedad, A. Tatarodlu, Microelectron. Eng.2008, 85, 1495.[7] Ş. Altindal, F. Parlaktürk, A. Tatarodlu, M. Parlak, S. N. Sarmasov,A. A. Agasiev, Vacuum 2008, 82, 1246.[8] M. M. Bülbül, S. Zeyrek, Ş. Altindal, H. Yüzer, Microelectron. Eng.2006, 83, 577.[9] A. Tataroglu, Ş. Altindal, M. M. Bülbül, Microelectron. Eng. 2005, 81,140.[10] C. Y. Zhu, C. D. Wang, L. F. Feng, G. Y. Zhang, L. S. Yu, J. Shen, SolidState Electron. 2006, 50, 821.[11] R. Gharbi, M. Abdelkrim, M. Fathallah, E. Tresso, S. Ferrero, C. F. Piri,T. Mohamed Brahim, Solid State Electron. 2006, 50, 367.[12] C. D. Wang, C. Y. Zhu, G. Zhang, J. Shen, L. Li, IEEE Trans. Electron.Dev. 2003, 50(4), 1145.[13] E. Ehrenfreund, C. Lungenschmined, G. Dennler, H. Neugebauer,N. S. Sariciftci, Appl. Phys. Lett. 2007, 91, 012112.[14] O. Pakma, N. Serin, T. Serin, Ş. Altindal, Semicond. Sci. Technol. 2008,23, 105014.[15] W. A. Hill, C. C. Coleman, Solid State Electron. 1980, 23, 987.[16] E. H. Nicollian, J. R. Brews, MOS (Metal Oxide Semiconductor) Physicsand Technology, Wiley: New York, 1982.[17] E. Arslan, Ş. Altindal, S. Özçelik, E. Ozbay, J. Appl. Phys. 2009, 105,023705.[18] J. Jiang, O. O. Awadelkarim, D. O. Lee, P. Roman, J. Ruzyllo, SolidState Electron. 2002, 46, 1991.[19] R. Castagne, A. Vapaile, Surf. Sci. 1971, 28, 157.[20] B. Akkal, Z. Benemara, A. Boudissa, N. B. Bouiadjra, M. Amrani,L. Bideux, B. Gruzza, Mater. Sci. Eng. B 1998, 55, 162.Surf. Interface Anal. 2010, 42, 812–815 Copyright c© 2010 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/sia

Temperature-dependent profile of the surface states and series resistance in (Ni/Au)/ AIGaN/AIN/GaN heterostructures

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Temperature-dependent profile of the surface states and series resistance in (Ni/Au)/ AIGaN/AIN/GaN heterostructures

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